Atherosclerosis of the lower extremities, also known as peripheral arterial disease (PAD), is a highly prevalent condition affecting about 5% of adults over 50 years of age and about 15% of adults over 70 years of age in the United States. A typical symptom of PAD is pain in the legs during exertion that is relieved with rest. However, other degrees of PAD are possible ranging from mild to severe. Patients with PAD may be entirely asymptomatic.
One method of diagnosing PAD is to compare the blood pressure values from two patient extremities in the upper and lower limbs (e.g., leg and arm blood pressure values). This method is generally referred to herein as an extremity blood pressure ratio (EBPR) examination. The most common EBPR examination for the diagnosis of PAD is the ankle brachial index (ABI) examination. The ABI exam compares a blood pressure value from the brachial artery in a patient's arm with a blood pressure value from the patient's ankle. When the ankle and arm systolic pressure values are obtained, the ratio of ankle pressure to arm pressure is normally greater than 1.0. An ankle/brachial ratio (ABI) that is 0.9 or less is considered abnormal and indicates the presence of significant PAD in the patient. The ABI value may reflect the severity of PAD in the index limb (for example ABI values of 0.9-0.7 are consistent with mild disease, ABI values of 0.7-0.4 are consistent with moderate disease, and ABI values of less than 0.4 are consistent with severe PAD).
A variety of techniques may be used in order to determine blood pressure values for an EBPR procedure. For example, a sphygmomanometer (e.g., a blood pressure cuff) may be placed around an extremity of the patient and inflated to occlude blood flow through an artery. A trained health care provider may, for example, use a stethoscope to listen for Korotcoff sounds associated with the return of blood flow in the artery during deflation of the blood pressure cuff to determine systolic and diastolic pressure within the artery. Other techniques to determine blood pressure values have been developed that may employ a Doppler ultrasound blood flow detector rather than a stethoscope to determine when blood flow returns to the artery (systolic pressure). However, the use of a Doppler ultrasound blood flow detector still requires trained health care providers in order to operate the detector correctly.
Another technique for determining an indicated blood pressure value includes obtaining a pressure signal that oscillates in a manner that corresponds to fluctuations in limb blood volume during deflation of a blood pressure cuff as blood flow returns to an occluded artery. This signal may be referred to as an oscillometric signal. The oscillometric signal can be processed to obtain a mean arterial pressure (MAP) and indicated values of systolic and diastolic pressure. This practice has been widely accepted for measurement of blood pressure in the arms of patients in clinical practice. However, use of the oscillometric technique for determining blood pressures in the lower extremities of patients has been questioned, particularly in cases where the patient is suffering from some degree of PAD.